We propose to examine mechanisms whereby the hyperglycemia of diabetes mellitus and its resulting two metabolic perturbations, increased polyol pathway activity and decreased myo-inositor content in retinal capillary pericytes (mural cells) may result in retinal capillary pericyte loss. Loss of pericyte may impair the integrity of the retinal microvascularture, leading to the breakdown of blood-ocular-barrier. The biochemical mechanisms by which high glucose causes reduction of pericyte viability in vitro and in vivo and the role of these two metabolic perturbations are the principal objectives in the present application. Since the decreased cellular myo-inositol may cause a reduction of phosphoinositides, including phosphatidylinositol 4,5 bisphosphate (PIP2); and hydrolysis of PIP2 produces novel second messengers (inositol trisphosphate (IP3) and diacylglycerol (DG)) which are implicated in cell proliferation, it is likely that alterations in myo-inositol metabolism induced by high glucose may contribute to a decrease in the rate of retinal capillary pericyte proliferation. In the proposed investigation we will study the effect of high glucose on the inositol metabolism. The production of phosphoinositides and second messengers (IP3 and DG) will be quantitatively determined. Myo-inositol synthase or PIP2 phosphodiesterase activity which may be important for maintaining myo-inositol or PIP2 levels respectively, will be assayed. Under "normal" and high glucose conditions the incorporation of tritium-labelled thymidine, an index of cell proliferation, and cellular sorbitol and myo-inositol levels will be simultaneously determined. In the proposed studies, the normalization of high glucose or decreased myo-inositol by insulin or myo-inositol supplementation in culture medium, respectively, and reversal of elevated polyol pathway activity by aldose reductase inhibitors may be employed as in vitro "therapy" to correct the decreased rate of retinal capillary pericyte proliferation.